A close friend rationalized their racism by asking this question in different words and then made alarming suppositions. I was taken aback and stammered while excusing myself.
There's vitriol behind this question that isn't mine. I apologize. I'd like to be able to explain what has held them back and why they aren't as developed as, say, Japan.
Have you checked out this FAQ section? It's not perfect, I know, but it might be worth using as a basis.
Nathan Nunn published a paper that claimed the slave trades ruined a lot of Africa's developmental potential in the following ways:
a. massive depopulation - Many people only know of the Trans-Atlantic slave trade but there were several (4 IIRC) major slave trade routes that occurred over the centuries in Africa. Nunn contends that the number of slaves exported from each African country inversely corresponds to its current economic performance.
b. political destabilization - Neighboring tribes/ethnic groups that were or would have been in the process of unifying were broken apart and turned against one another. Some groups such as the Dahomey came to prominence by working as slave raiders. Sometimes, the same ethnic group turned against its constituents.
http://scholar.harvard.edu/files/nunn/files/the_long_term_effects.pdf